When an aircraft flies through cold moist air, ice can build up on the aircraft wing leading edges and other aerodynamic surfaces. Ice accretions can alter the airflow over the wings and tail, reducing the overall lift force and potentially causing aerodynamic stall—a condition that can lead to temporary loss of control of the aircraft.
The imminent threat of aerodynamic stall due to icing and other factors has led regulatory agencies, such as the Federal Aviation Administration (FAA), to require aircraft manufacturers to incorporate methods, systems, and devices into aircraft to protect against and prevent both (a) excessive icing and (b) aerodynamic stall.
Ice Prevention
Ice can be visually detected by the pilot, but most modern aircraft have one or more ice detector sensors that warn the flight crew of icing conditions. In some aircraft models, multiple ice detectors are used, and the ice detection system automatically turns on anti-ice systems and modifies stall warning and stall protection systems when ice is detected.
There are many ways to remove and/or prevent ice buildup during flight. For example, certain Embraer aircraft use hot bleed air from the jet engines to melt or prevent ice formation on the wings and other surfaces. See e.g., Embraer 135/145 Ice and Rain Protection.
Stall Warning and Protection Systems
A Stall Warning System warns the pilot, often audibly and visually, that the aircraft is on the verge of entering aerodynamic stall. As explained in detail below, an automatic system can compare certain measured flight parameters (i.e., angle of attack) to a threshold (i.e., maximum angle of attack setting) to determine when an aircraft is close to aerodynamic stall. Some aircraft warn the pilot of an imminent stall by rapidly shaking the yoke or stick. If the pilot takes no immediate action, then a processor or other control circuit may actuate some servo on the control column to force the nose of the aircraft down to prevent stalling.
A Stall Protection System uses flight envelope protection, which is an automatic control system that actuates the aircraft flight control surfaces to prevent the pilot from making control commands that would force the aircraft to exceed its aerodynamic operating limits and enter a stalled region. This system also protects the aircraft from atmospheric disturbances (such as gusts and wind shears) that could drive the aircraft into the stalled region.
Stall Warning/Protection Based on Icing
Stall Warning and Stall Protection Systems are also operative while flying in icing conditions. When any ice detector detects ice, an anti-icing system is activated (usually automatically) to melt the ice and prevent further ice formation. The ice detectors also send a signal to a Stall Warning or Stall Protection System, which reduces its maximum angle of attack setting to prevent the airplane from flying into the stalled region when the aircraft aerodynamic characteristics is degraded by ice.